The present invention relates to improved commutation and control of multi-phase to single-phase AC--AC power converters and their utilization as AC floating power cells or modules in high-power electronic systems and AC motor drives of either single-phase or multiple-phases. In particular, the present invention provides an improved power circuit architecture of basic converter building modules for modular AC--AC power conversion, adaptive control for safe and effective phase commutation, snubber configurations, and new configurations for integrated and intelligent multiple AC-switch power modules. These techniques and configurations are provided in a systematic approach for designing optimal AC--AC converter systems
Existing AC--AC power converters have been developed based on a three-by-three (3.times.3) matrix circuit configuration. Two illustrations of such a converter and bi-directional power switches are illustrated in FIGS. 1(a) and 1(b). FIG. 1(a) shows a power circuit for matrix converter which converts a 3-phase AC power input to a variable-frequency and variable-voltage three-phase power output. FIG. 1(b) depicts a circuit configuration of a bi-directional semiconductor switch which consists of a diode bridge and a switching device T1, such as an IGBT or other power semiconductor device. While the diode bridge steers current from one direction to another, the center switching device, T1, controls the on-off of the load current path. This type of bi-directional switch has the advantage of only requiring one switching device per AC switch. However, there are two main disadvantages: (1) Three devices are conducting at any time in an AC switch, resulting relatively high conduction losses. (2) The switching device T1 must directly interrupt the load current by "hard switching", since the switch configuration provides no possible alternative path for load current commutation in FIG. 1(b), unless an additional auxiliary commutation circuit branch is provided.
In fact, the power converter in FIG. 1(a) must have a dead-time insertion during switch commutation. The rapid change in current in the outgoing phase causes an electromagnetic interaction with the equivalent input inductance, L.sub.a, which is in serial connection with the outgoing device. This generates a large voltage spike, L.sub.a di/dt, across the outgoing power semiconductor device. It therefore often requires excessive snubber circuits for the power converter in FIG. 1(a) to prevent the devices from the harmful over voltage stresses. The problem becomes more difficult to handle as the power rating of the converter increases.
An alternative commutation method is to have a brief overlapping conduction of both incoming and outgoing phases to maintain a constant load current flow during commutation. However, this would provide a short circuiting current path between two input power phases and undesirable large circulating current could be produced.
Power electronic engineers have faced major challenges to address the shortcomings of the prior art and to provide a practical high power AC--AC converter designs which are suitable and cost effective for manufacturing of high-power systems. The key technical issues are listed as follows:
(1) The need of practical AC power switches which can reduce the converter conduction losses and provide flexible bi-directional gate control for better circuit commutation. PA1 (2) Improving commutation techniques to overcome the disadvantages of hard switching associated with the matrix converter in FIG. 1(a) as aforementioned. PA1 (3) Reducing complexity of the control and power circuit by possible integration of commutation controls and multiple power switches into single device modules. PA1 (4) Employing modularity system design techniques, including optimal partitioning and system architecture, which allows high power AC--AC conversion systems to be designed and implemented based on standardized basic power building modules using switching devices and passive components which have lower power rating to achieve cost reduction.